Emission concerns associated with the operation of internal combustion engines (e.g., diesel and other types of engines) have resulted in an increased emphasis on the use of exhaust gas heat exchangers. These heat exchangers are often used as part of an exhaust gas recirculation (EGR) system, in which a portion of an engine's exhaust is returned to the combustion chambers. Such a system displaces some of the oxygen that would ordinarily be inducted into the engine as part of the fresh combustion air charge with the inert gases of the recirculated exhaust gas. The presence of the inert exhaust gas typically serves to lower the combustion temperature, thereby reducing the rate of NOx formation.
In order to achieve the foregoing, it is desirable for the temperature of the recirculated exhaust to be lowered prior to the exhaust being delivered into the intake manifold of the engine. In the usual case, engine coolant is used to cool the exhaust gas within the exhaust gas heat exchanger in order to achieve the desired reduction in temperature. The use of engine coolant provides certain advantages in that appropriate structure for subsequently rejecting heat from the engine coolant to the ambient air is already available for use in most applications requiring an EGR system.
Due in large part to the elevated temperatures of the exhaust gas that they encounter, exhaust gas coolers are known to be prone to thermal cycle failure. The desire for increased fuel economy continues to drive the engine operating temperatures upward, further exacerbating the problem. Above a certain temperature, the material properties of the metals used to produce the heat exchanger rapidly degrade, and the operational lifetime of the heat exchanger is substantially reduced. In order to combat this problem, it often becomes necessary either for the heat exchanger to be produced of more expensive alloys that can withstand these higher temperatures, or to increase the size and weight of the heat exchanger using the current materials, neither of which is desirable. Thus, there is still room for improvement.